This invention relates to methods for casting superalloy articles. More particularly, this invention is directed to a method for processing an article cast from an oxide scale-forming superalloy, in which the sulfur content of the superalloy is reduced so as to result in the article exhibiting improved environmental resistance.
Higher operating temperatures of gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase, particularly those engine components subjected to the most severe thermal environments, including the first and second stage high pressure turbine airfoils, first and second stage nozzles, and shrouds. Significant advances in high temperature capabilities have been achieved through the formulation of nickel, iron and cobalt-base superalloys whose mechanical properties at elevated temperatures are enhanced when produced in the form of a single crystal or directionally solidified casting. Even so, such advanced superalloys alone are often inadequate for components to survive the severe thermal and oxidizing environment in the turbine, combustor and augmentor sections of a gas turbine engine.
A common solution is to form a protective layer on such components in order to minimize their service temperatures and enhance their environmental performance. For this purpose, superalloys have been formulated to develop a metal oxide surface scale that forms a stable and environmentally-resistant barrier layer on the surface of the superalloy. In addition, thermal barrier coatings (TBC) of ceramic materials have also been developed that tenaciously adhere to the oxide layer on the surfaces of the superalloy. To be effective, such protective layers and coatings must be strongly adherent to the component and remain adherent through many heating and cooling cycles. This requirement is particular demanding due to the different coefficients of thermal expansion between the oxides and ceramic materials that form the protective layer and the superalloy materials that form the turbine engine components.
Though advances have been made, a continuing challenge has been to achieve more adherent oxide layers and thermal barrier coatings that are less susceptible to spalling. It is known that spallation is encouraged by the presence of sulfur within a superalloy. When the superalloy is heated, the sulfur segregates to the critical oxide-metal interface and weakens the chemical bond strength of the interface, thereby permitting spallation of the oxide layer and the thermal barrier coating (if present) and depleting the superalloy of critical scale-forming elements such as aluminum and chromium. Therefore, efforts have been made to either reduce the sulfur content of superalloys or prevent sulfur from segregating to the oxide-metal interface. Such efforts have included adding an oxygen-active element such as yttrium to the superalloy composition, thereby forming a stable sulfide that remains dispersed in the bulk alloy. Alternatively, the amount of sulfur in a superalloy composition can be held to levels that are sufficiently low, generally about one part per million by weight (ppmw) or less, to avoid the deleterious effect of sulfur segregation to the oxide-metal interface.
Methods for achieving low levels of sulfur in a superalloy are typically characterized as expensive or ill-suited for mass-produced superalloy components,;such as airfoils, nozzles and shrouds. For example, though hydrogen annealing techniques have been shown to reduce sulfur content to as little as 0.2 ppmw, such techniques require long anneals at high temperatures in a reducing environment that poses a significant hazard. Alloy processing techniques by which sulfur is reacted with rare earth metals have proven to be feasible, but additional reactions occur that permit sulfur to be reintroduced into the metal.
In contrast to the above, methods by which yttrium is added to cast superalloy components are relatively developed. Yttrium is typically chosen over other oxygen-active elements because of its solubility, higher relative eutectic temperature with nickel, and lower relative cost. Yttrium is typically added in an amount that is larger than that required to tie up the sulfur within the superalloy because some yttrium is lost to evaporation, while the remaining yttrium tends to react with the ceramic molds and cores used in the casting operation. An example of the latter is the reaction of yttrium with silica-containing molds and cores widely used to investment cast superalloy components:
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To prevent the removal of yttrium through such a reaction, facecoats for ceramic molds and cores have been developed that are nonreactive with yttrium in the superalloy melt. Facecoats formed of yttria (Y2O3) are widely employed since they contain yttrium in its most stable state, though other very stable oxide compounds could be used as facecoat materials.
Though the use of such facecoats enables sufficient yttrium to remain in the superalloy melt and bind the sulfur within the melt, the relatively high levels of yttrium required are not always desirable in terms of the desired properties for the superalloy. Accordingly, it would be desirable if other methods were available that prevented the deleterious effect of sulfur segregation at the oxide-metal interface of a superalloy article and were conducive to mass-produced superalloy components, yet avoided the requirement for high levels of rare earth metals within the superalloy composition.
It is an object of this invention to provide a method for forming a superalloy article on which a protective oxide scale is developed to promote the environmental resistance of the article.
It is a further object of this invention that such a method entails processing steps that prevent spallation of the oxide scale caused by the segregation of sulfur to the interface between the oxide scale and metal substrate.
It is still a further object of this invention that such a method eliminates the requirement to include relatively high levels of oxygen-active elements in the superalloy for the purpose of tying up sulfur and preventing its segregation to the oxide-metal interface.
It is yet another object of this invention that such a method does not require long processing times, such that the method is conducive to high volume production.
The present invention provides a method for promoting the environmental resistance of articles cast from nickel, iron and cobalt-base superalloys of the type alloyed to develop a protective oxide scale, including various alloys used in the production of high pressure turbine airfoils, nozzles and shrouds. The method entails removing sulfur during or subsequent to the casting operation, and therefore does not rely on techniques that remove sulfur from the superalloy melt or require high levels of an oxygen-active element within the superalloy.
The method generally includes casting a superalloy article in a mold cavity, and then heat treating the article while the surfaces of the article are in contact with a compound containing a sulfide-forming element. As used herein, sulfides encompass sulfides, oxysulfides and other sulfide compounds that may form as a reaction product of sulfur in the article. The heat treatment is performed at a temperature sufficient to cause sulfur within the superalloy article to segregate to the surfaces of the article, which enables the sulfur to react with the sulfide-forming element and thereby form sulfides at the surface of the compound. The compound is then separated from the surfaces of the article so as to simultaneously remove the sulfides and any elemental sulfur that have segregated to the surface of the article. Advantageously, because sulfur is removed with the compound, additional processing or surface treatments of the article for sulfur removal are unnecessary.
In one embodiment of the invention, the surfaces of the mold cavity are coated with the compound containing the sulfide-forming element, and the heat treatment is carried out while the article is within the mold cavity. In this manner, separation of the compound entails removing the article from the mold cavity, during which sulfides and elemental sulfur at the surface of the article are simultaneously removed. In another embodiment of this invention, the compound containing the sulfide-forming element is deposited as a coating on the article after the article has been removed from the mold cavity and prior to the heat treating step. After heat treatment, the compound is removed from the surfaces of the article by a chemical or mechanical process.
According to this invention, one or more compounds containing a sulfide-forming element can be used in combination, examples of which include yttria (Y2O3), calcium oxide (CaO), magnesia (MgO), scandia (Sc2O3), ceria (CeO2), hafnia (HfO2), zirconia (ZrO2), titania (TiO2) lanthana (La2O3), alumina (Al2O3) and silica (SiO2). In addition, heat treatments can be performed subsequent to removal of the sulfides, as may be desired to stress relieve, age or otherwise improve the mechanical properties of the superalloy article.
The method of this invention results in a superalloy article characterized by enhanced environmental resistance as a result of sulfur being removed during the manufacture of the article. Specifically, the oxide scale and any thermal barrier coating employed to form a protective barrier on the surface of the article are less susceptible to spalling as a result of sulfur segregation being prevented. Advantageously, the method does not require long processing times or special fixtures, additional alloying constituents that might alter the properties of the superalloy, or materials that are expensive or difficult to obtain. As a result, the method is highly conducive to use in the manufacture of relatively high volume components, such as airfoils, nozzles and shrouds of gas turbine engines.
Other objects and advantages of this invention will be better appreciated from the following detailed description.